1. Field of the Invention
The present invention is directed to systems and methods for monitoring the mechanical health and status of aircraft landing gear, and more particularly to systems and methods for monitoring the loads applied to an aircraft's landing gear structure, and still more particularly to landing gear monitoring systems that vary the sensor sampling rate and digital data resolution in order to conserve power during on-ground monitoring when the aircraft's power bus is shut down and a secondary energy source is used to power the monitoring system.
2. Description of the Related Art
Landing gear overload is a condition when the mechanical stress experienced by the landing gear structure exceeds or comes unacceptably close to the design limits. Existing or previously proposed landing gear monitoring systems focus on the detection of overload conditions due to hard landing events. In most aircraft currently in operation, a hard landing declaration is made by a pilot based on his or her subjective judgment.
Following a hard landing declaration, a series of inspections are undertaken to ascertain which components may have been damaged or excessively stressed. Such inspections are complicated, time-consuming and typically lead to significant loss of revenue by the aircraft operator. The overwhelming majority of hard landing declarations are later determined to be unreliable at predicting the mechanical health of the landing gear.
In some aircraft, recorded flight data such as, for example, altitude, velocity, and rate of descent may be used to assess the actual severity of the landing event. For example, U.S. Pat. No. 7,589,645 to Schmidt discloses an overload detection system that uses accelerometer measurements combined with flight data recorded by the avionics system to produce a hard landing diagnosis. Such a method is highly inaccurate, as it does not give sufficient information about the actual stresses experienced by the individual landing gear assemblies. Accuracy of this approach is limited by the fact that the acceleration experienced by the airframe does not translate directly into stresses experienced by individual landing gear assemblies.
Therefore, there is a need for a sensor-based system to objectively and accurately assess the degree to which the stress levels may have approached the design limits. Such an overload detection system would be able to confirm or disprove a hard landing declaration made by a pilot. If such a decision is made accurately and reliably, the operators may minimize flight operations and maintenance costs, while still assuring safe aircraft operation.
A novel system for predicting the forces applied to landing gear has been developed by the assignee of the present invention and has been described in a U.S. Provisional Patent Application No. 61/455,169. This system overcomes the limitations of the accelerometer-based monitoring by measuring mechanical strain in one or more locations on the landing gear, which are selected in such a way that the measured strain data, when taken collectively, allows determination of actual forces applied to the gear. An advantage of this approach is that it provides overload detection separately for each landing gear assembly. This is in contrast to the accelerometer and avionics-based methods, in which it is difficult to differentiate between the effects on left and right gear, for example. Furthermore, the new approach simplifies the inspection procedure by providing information as to which landing gear elements may have suffered damage.
Yet another benefit of the new method is that it is also applicable to low-velocity overload cases occurring during ground maneuvers, which are typically not associated with high acceleration levels. In contrast, the accelerometer-based method is limited to landing-related (high velocity) overloads. One of the leading causes of landing gear damage or excessive fatigue is towing-related overload. The nose landing gear is particularly susceptible to damage while the aircraft is being towed. This issue is known to the operators as ‘tug abuse’. If the towing truck crew does not strictly follow the aircraft towing procedures, the landing gear may be subjected to large stress levels without the aircraft crew's knowledge. The new monitoring system discussed above allows detection of such overload cases by estimating the actual mechanical load on the landing gear during the ground operations. The system saves the stress data and provides the flight deck crew with information about any recent possible overload events. This offers a solution to the tug abuse detection problem.
However, a practical complication to the tug abuse monitoring issue is the power supply problem. It occurs quite frequently that an aircraft will be towed when all its electrical systems are powered down. This may occur when an aircraft has been parked, perhaps for a few days, and is being towed to another area of the airport, for example to be put into service again. In such cases there may be no flight crew present and any possible landing gear overload could go unnoticed. If the monitoring system operates solely from the aircraft power, then it will be inoperative and miss such cases. An alternative is to operate the system using a dedicated battery. This however means that the length of the system operation may be limited by the practical battery size and weight. In order to successfully detect overload cases that cannot possibly be predicted beforehand, the system must monitor the measured signals continually without any prolonged sleep times. Continuous operation of multiple sensors may require a significant power draw, limiting the useful battery life. If the battery is depleted and the system stops operating before the towing starts, any overload event will go undetected.
Thus, there is a need to design a low-power system for monitoring loads applied to landing gear when the aircraft is on the ground and its power bus is shut down.